The present invention relates to a loudspeaking telephone for providing hands-free communication and, more particularly, to a voice-switched loudspeaking telephone which controls the gain of transmitting and receiving paths by comparing the level of a transmitting path signal (a signal to be transmitted on a transmitting path) and the level of a receiving path signal (a signal received on a receiving path).
In a telephone of the type described, a sending voice signal is inputted through a microphone and is amplified by a microphone amplifier and, then, is applied to a variable loss circuit (variolosser) to be controlled in level. The output of the variolosser is fed out to, for example, a subscriber line via an output amplifier and a hybrid circuit.
On the other hand, a receiving speech signal from the subscriber line is routed through the hybrid circuit and an input amplifier and, then, is controlled in level by a variolosser which independent of the previously mentioned one. The output of this variolosser is amplified by a speaker amplifier, and, then outputted through a loudspeaker as the sound of a received voice signal. The input to the variolosser on the input side and the output from the variolosser on the output side, i.e., the outgoing and incoming signals, are compared by a level comparator with respect to their level. Based on the result of this comparison, the level comparator controls the amount of insertion loss to be effected by the variolossers which are respectively located on the input side and the output side. The amounts of the insertion loss should be large enough to prevent "singing", as described hereunder.
In the above example, a balance network is installed for the impedance balancing of the hybrid circuit. In practice, however, a certain amount of runaround occurs from the transmitting path to the receiving path, i.e., sidetone coupling. In addition, the received voice which signal is outputted through the loudspeaker is coupled to the microphone, an occurrence generally referred to as acoustic coupling.
It follows that, when the amount of insertion loss introduced by the variolosser is reduced on the transmitting and receiving paths, the probability is increased that the gain becomes greater than 1 in the loop which extends from the microphone and back to the the microphone via the hybrid circuit and the speaker, thus causing singing.
To prevent such singing, the gain on one or both of the transmitting and receiving paths is reduced to a level that is lower than the levels which are particular to the each path. The amount of decrease in gain, i.e., the amount of insertion loss in the variolosser, depends upon the amount of acoustic coupling and of sidetone coupling. Generally, the amount of acoustic coupling involves substantial peaks and dips with respect to the frequency characteristic and noticeably fluctuates depending upon the environment in which the telephone is situated. Hence, the insertion loss should be great enough to guarantee stable operation while preventing singing. This signing prevents a two-way communication from being held smoothly.
With respect to the details of the above-described telephone, reference may be made to the following literature:
(1) The 4A Speakerphone-Hands-Down Winner, Bell Laboratories Record, pp. 233-237, Sept. 1973 PA1 (2) Functional Design of a Voice-Switched Speakerphone, The Bell System Technical Journal, pp. 649-668, May 1961 PA1 (3) Fundamental Considerations in the Design of a Voice-Switched Speakerphone, The Bell System Technical Journal, pp. 265-293, Mar. 1960 PA1 (4) Loudspeaking Telephone Circuit, U.S. Pat. No. 3,952,166.